Field of the Invention and Related Art Statement:
This invention relates to heavy duty pneumatic radial tires, and more particularly to an improvement in the belt structure of the heavy duty pneumatic radial tire.
The radial tire of this type has a casing reinforcement comprising a carcass composed of at least one cord ply containing cords arranged in a substantially radial plane of the tire and a belt composed of at least three rubberized steel cord layers superimposed about the carcass just beneath a tire tread. In such a tire, there is widely used a belt structure , wherein a first belt layer close to the carcass, a second belt layer having a width somewhat wider than that of the first belt layer and a third belt layer having a width substantially equal to that of the first belt layer these layers are superimposed one upon another in such a manner that a cord angle defined as an intersection angle of cord with respect to the mid-circumference of the tire is smaller in the second and third belt layers than in the first belt layer and the cords of these belt layers are crossed with each other with respect to the mid-circumference. However, such tires have a drawback that the cord breaking of the belt inside the tire is apt to be caused by stress concentration based on ruggedness such as stones scattering on road surface, protrusions or the like.
For this end, it has been attempted to mitigate stress by arranging the first belt layer as a pair of belt layer portions separated widthwisely from the mid-circumference of the tire or by a so-called middle removed arrangement. In this attempt, however the reinforcing action of the belt in the middle portion of the tread is weakened, so that the stability of the tread is insufficient and the strain between the second and third belt layers becomes large. This is apt to cause separtion failure at the belt end.